Reflectometer



Sept. 21 1926. 1,600,865

F. A. BENFORD REFLECTOMETER Filed Sept. '7, 1923 9- Hagfltto'r'eg.

Patented sept. 21, 192e.

UNITED STATES PAgrr-:N'r OFFICE.

FRANK 'AQ BENFORD, OF SCHENECTADY, NEW YOB-K, ABSIGNOR TO GENERAL ELEO- v TBIC COMPANY, A CORPORATION OF NEW YORK.

aarnncromnrnn.

Application led september 7, 1923*. Serial No. 661,499.

My invention relates to reecto'meters, that is, devices for measurlng the amount of light reected by a surface. )ne form of device for this purpose whlch 1s on the market employs two parallel surfaces between which an area is bounded b a specular ring. In this device, one o the sur-v faces on one side of the ring serves as the illuminating surface and the surface on the opposite side of the ring, as the illuminated surface. Through the medium of a photometer, a region of the illuminating surface is compared with a region of the illuminated surface. Devices of the jabove character are usually portable. The typeof devlce of the prior art has the objection that 1t 1s not accurate by approximately ten per cent. The causes of the inaccuracy are, for example, the use of a specular surface as the bounding surface between the two planes; also the large extent ofthe bounding surface; also, the angle of observation 1s not the best angle for obtaining an average read- Among the objects of m invention are to provide a refiectometer w ich is free from thev above objections and one which is ac-4 curate to a much greater extent and to elimi- -nate as much as possible the bounding surfaces between the planes. This is done by causing the surfaces to slope with respect to each other and by reducing the sloping angle as much as ossible while preserving the proper angle o? observation between the hotometer and the surfaces and by using.

1n this connection reflecting prismsbetween the lreflectometer and the surfaces.

Another object is to provide other details of improvement for increasing the eiciency and serviceability of a device of the4 above character. The means for accomplishing the above and other, useful'ends are hereinafter more fully set forth and claimed, reference being had to the accompanying sheet of drawings, Fig. 1 of which shows in perspective, the photometer attached to the bounding surface or frame located between the two lsurfaces and which is substituted for the specular ring of the old type of device.. In this figure, the reflecting prisms within the frame are also shown. In Fig. 2 there is shown a vertical section of the complete device taken on line 2-2, Fig. 1. 'In Fig. 3 are shown curves representing readings obtained in connection with the diffusing surface and thefspecular surface when the field of observation is moved toward or away from the apex of the dihedral angle formed by the two planes. In the portab e reflectometer ofthe art the centers of te fields of observation have always beenfso located with respect to the limiting bounds between the two surfaces that the readings of both specular and diffusing surfaces, such as glass and ma nesium ca'rbonate respectively, have been low. With the device Aof'my construction, the nearer the field of observation is selected to the apex of the dihedral angle between the planes, in determining the reection of the diffusing surface,the higher the reading. The reverse'is the case when a specular surface is used. At points very near the apex of the angle, the readings for a specular surface are too low and as the region is selected further away from the angle, the

curve rises so that a point is reached bee yond ,which the readings" are too high. I therefore selected the points -A and B which represent the centers respectively of the fields of observation on the illuminating surface and on the illuminated surface ata distance from the apex C of the dihedral angle where the photometric error for the specular surface is about 1 per cent lhigh and for the magnesium carbonate, about 1 per cent low.

Referring more in detail to the drawings the photometer P is of the standard type and 1s provided with an objective leus 3, polarizer 4, analyzer '5, and eye-piece 6. This photometer is secured to the frame 7, Fi 1, in any suitable manner, the forward and of which photometer projects through the side piece 7 of the frame. The frame is adapted to receive on top thereof the translucent plate 8. .For thiis purpose guides 9 are provided which guides are secured to the sides of the frame. When in position, the forward edge reflector 13 along the line 14 which reflector directs the light throuhout the lens 3 of the Photometer and fina ly reaches the eyeieee 6. Similarly, the 'photometer receives ight from the lower surface 12 from the region about the point B, both of vwhich points are in the same plane. The light from the point B reaches a second prismaticxeflector 14 along the line 15, which reflector directs the light to the lens 3 through which the light finally reaches the eyepiece 6. Both of the prismaticreflectors 13 and 14 are suitably mounted on springs 15 and 16 which springs are secured to the forward end of the hotometer.- I have found that in order to obtain a correct readiiig, the angle between the lines 14 and 15 and the surfaces 8 and 12 respectively should be of a particular magnitude, that is, -25 degrees more or less. In order to obtain this angle without the use of the prismatic reflectors 13 and 14, it will be necessary to increase the angle between the surfaces 8 and 12 which would increase the area of the frame 7. This is undesirable inasmuch as the effect of increasing this surface is t0 decrease the efficiency of the device in .the matter of correct readings. In this connection, it

the frame 11 so as to cause the sides to slope as shown in the drawings, the area of the vframe is so reduced that the error in the readings is decreased to 5 per cent. Furthermore, I find that by moving the points A and B toward the apex C of the angle between the plates, I can reduce the error con' siderably to a value in the neighborhood of 21/2 per cent. Furthermore, by making the inner surface of the frame of a white diffusing character, as for example, by the use of White enamel, the error inthe readings may be still further reduced so that the readings of the instrument may be in the neighbor-` hood of 1/2 of 1 per cent of being exact.

Passing now to a technical consideration of the invention, it can be readily demonstrated that if a plane surface, such as surface 12, is illuminated by a parallel surface of infinite extent, the illumination of the plane surface will be equal in brightness to that of the illuminating surface. However, a plane of infinite extent is not to be attained in practice `and it is therefore necessary to resort to somepractical device'which will approximate the same result. Overlookingfor the moment the importance of obtaining ideal conditions, lit will be assumed that the transparent porcelain plate which maybe used as the upper surface of the dihedral angle shown in Fig. 2,l is the illuminator 4or source of light for the surface 12` upon which the instrument rests and which 1s to be tested for its coeiiicient of reflection. lVhen these two.

surfaces 8 and 12 are observed through the optical train that forms a photometric field illuminatedby polarized light, the

ulpper half of the field is found to derive its lght from the'point A on the end surface of the -illuminator and the lower half ofthe field is illuminated from the point B on the test surface 12. By rotating the Nicol prism 5 contained in the rear of the optical tube, the intensity of one field may be raised while the other is lowered and the photometric balance the Nicol prism, will be at 45 degrees, or at ,i

the corresponding position 135 degrees, 225 degrees and 315 degrees in the other quadrants. If the test surface falls short of be,- ing perfectand .it always does-the balancing point will fall short of 45 degrees, or exceeds it," depending of the polarized light is bein measured. The scale of rotation is marked in degrees andfor one component of a polarized light Wlith the scale in the position shown in Fig. 1, the relative brightness of B to A is =tan2 a Where a .is the departure lof the pointer of the vertical axisvof thel scale and where the ent-ire optical train, except the prisms within the dihedral space is turned through degrees of the other polarizer component is l where a is now greater than 45 degrees. The coefficient K of the surface`12 is K.=1

The illuminator, not being perfect, certain sources of error must be considered. It is supposed that only the point B on the test sur ace is being considered. In the instrument, as constructed, the direct illumination for the sloping illuminator 8 is some 96 per cent of an infinite parallel illuminator and by coa-ting the surrounding walls of the frame 9 with a ood white enamel, it has been found possilile to bring the illuminator up to 99.6 per cent of the. theoretical magnesium, whereas, the reflectometer of the prior art previously mentioned can reach only 90 per cent of the magnesium and its readings are at best lsome 10 per cent low. Thus, magnesium carbonate, whose coefficient has been determined b a number of experimenters to have a coe cient of 0.98, has, according to the instrument of the prior art mentioned, a coefficient of .98, and as determined by the instrument of my invention, has a coefficient of 0.974. This test surface is a very good diffuser, that is, under a beam of light upo'n which component the surface brightness is almostgequal from any angle of view. When testing specular surfaces such as a mirror, there 1s another factor entering the action of the instrument and it will be shown how this factor can be used to counterbalance certain other factors and thus provide and thus arrive at a high degree of accuracy.

It has been found experimentally that often when the illuminator derives its light from a distant source and is originally of the most uniform brightness, the edge of the illuminator near the point of the edge is slightly brighter than this part near' the back wall through which the optical train enters. As a result, the point A1 is slightly brighter than the point A and since the image of A1 is observed in the mirror, there is a decided tendency for the instrument to give readings above the true value. We thus have an instrument that for diffusing surfaces reads low by less than 1 per cent and it has been found experimentally that the readings of aspecular surface are less than 1 per cent high. The great majority of surfaces reflect light by a mixture of diffused and specular action and in this most common of all cases, the positive error of the specular component largely overcomes the negative error of the diffused components and in any event, the error can be said to lie within the limits of i1 per cent.

It is probably true that the coefcient of reflection of every surface is different for each angle of observation, thus, magnesium carbonate under diffused illumination, such as oxides under 'the illuminator of my invention, will be slightly brighter when viewed along a line nearly grazing the surface than if viewed along a line normal to the surface. A sheet of ordinary glass under the same conditions would have a coeficient of brightness of nearly 1 at gazing or 0.08 when viewed from a line normal to its surface. We are here confronted withv the fact that the apparent coefficient of reflection of the surface depends upon the angle of view and it is therefore desirable to be ab'le to arrange the angle of view so as to obtain the particular angle 'that will give a fair average for all angles. In Fig. 2, the prisms within the dihedral angle may be rotated in a vertical plane and thus change both the angle of incidence and also the distance of lthe points A and B from the dihedral angle. This adjustment, in addition to a forward and backward adjustment of the entire optical train including the reflecting prisms 13 and 14, makes it possible to select a pair ofpoints A andB such that the positive and the negative errors will tend to cancel one another.

In order to more clearly illustrate what has just been said, the curves in Fig. 3 have been prepared and have drawnl with respect to Fig. 2, the abi C of the angle been y scissae indicatin the distance from the apex tween the` planes 8 and 12 to points on the test surface 12, such as point B. It willjbe seen therefore that the curve of the diffusing surface approaches and in fact reaches the magnesium, as the point B is shifted toward the right. On the other hand, the specular surface, as indicated by the vcurve rises further and further as the point B recedes from the point C and. crosses the zero line before the poin-t B reaches the4 point C. For this reason, the points A .and B have been located in proximity to the line a b, Fig. 3, which shows the specular curve and the diffusing curve .at about equal distances from the zero line.

What I claim as new and desire to secure by Letters Patent -of the United States is 1. In a device for measuring the coeflicient of reflection of a surface, a photometer, a diffusing screen and a frame for bringing the photometer, an area of t-he screen and an area of the surface to be tested into such rev lation that said areas fall within the field of the photometer for comparison and a refleeting element interposed between each of said areas and the photometer, along the path of the rays from each of said surfaces for directing said rays to the photometer and for increasing the angle of vision.

2. In a device for measuring the coefficient of reflection of a surface, a photometer, a diffusing screen and a frame for bringing the photometer, an area of the screen and an area of the surface to be tested into such re- 100 lation that said areas fall within the field of the photometer for comparison and a refracting lelement interposed between each of said areas and the photometer and along the path of the rays from each of said sur- 105 faces for directing the rays to the photometer and for increasing the angle of vision.

3. In a device for measuring the coeflicient of reflection of a surface, a photometer,

a diffusing screen and a frame for bringing 110 the photometer. an area of the screen and an area of the surface to be tested into such re`- lation that said areas fall within the field of the photometer for the purpose of comparison. and means 'for causing the rays from 115 said areas to cross twice in their path between the area and the eye-piece of the photometer.

4. In a device'for measuring the coefficient of reflection of a surface, a photometer, a 120 ldiffusing screen and a frame "for bringing the photometer, an area of the screen and an area of the surface to be te-ted into such. relation that said areas fall withintlie field of the photometer for the purpose of comparison. and means for causingr the rays from said areas to cross between said areas and the eyefpiece of the photometer.

5. In a device for measuring the coefficient of -reflection of a surface, a photometer, a ,|30

l thev photometer, fanarea` of the screen and an area of the surface to be tested into such relation l that they form a dihedral angle together with an enclosing and subtending bounding surface, a photometerprojected into said-angle and so related to the two surfaces that the area in each surface falls within photometer.

6. In a device for measuring the coefficient of reflection of a surface, a photometer, a diffusing screen and a frame for 'bringing the photometer. an area of the screen and an the field of the 'area of the surface to be tested into such re-l lation that they form a dihedral angle together with an enclosing and subtending bounding surface, .a photometer projected into said angle and so related to the two surfaces that the area in each surface falls Within the field of the photometer,'the center of either of said areas being located nearver to the apex of the angle than to the bounding surface along a plane perpendicular to the'illuininated surface and which also passes through the optical axis of the photometer.

7. In a device for measuring the coefficient."

of -refiection of a surface, a photometer, va dlffuslng screen and a frame for bringing thev photometer, an area ofthe screen and an area of the surface 'to be tested into sueh relation that they form a dihedral angle together Awith an enclosing and subtending bounding surface, a photometer projected into 'said angle and s o related to the two surfaces that the area in each surface falls within the field of the photometer, the center ofvv either of said areas being so located with respect to the apex of the dihedral angle that the reading for a specular surface is as much above the average for all readings from all angles with respect to said specular surface as the reading for a diffusing surface is below the average for all readings from all angles with respect to the diffusing surface.

'8. In a. device for measuring the coefficient of refiection of a surface, a photometer, a diffusing screen and a frame for bringing the photometer. an area of the screen and an area of the surface to be tested into such relation that they form a dihedral angle together with an enclosingjandl subtending bounding surface. a photomete'r projected into said angle and so related to the two sur- :faces that thc area in each surface falls within the field of the photometer. the centeriof either of said areas so located with respect to the apex of the dihedral angle that the reading fora specular surface is within one percent above the average for all readings from all of the angles with respect to said specular surface and the reading for a diffusing surface is within one percent below the average for all readings from all of the angles with V'respectto the diffusing surface. v

9. A device forv measuring the coefficient of refiectio'n of a surface, having a lphotometer, a diffusing screen and a frame for bringingthe photomete'r, the screen and the surface to be tested into such relation that the screen and the surface slope toward each other and toward the axis of the photometer to increase the angle, of observationthrough the photometer andto increase the dihedral angle of illuminationof the surface.`

\10. A device `for measuring the coefficient 0f`reflection of a surface, havingl a photometer, a diffusing screen and a frame for S0 bringing the photometer, thescreen and the surface to be tested into such relation that` thev screen andthe surface slope toward each other and toward the axis of the photometer to increase/che angle of observation through the-photometer and to increase the dihedral angle of illumination of the surface, said photometer and screen .being secured -to said frame, the Photometer belng so located that its axis projects into the dihedral angle formed between the screen andthe surface to vbe tested and between the screen on one side and the surface on the other.

11. A device for measuring the coefficient of reflection of a surface, having a photometer, `a diffusing screen and a frame for bringing the photometer, the screen and the surface to be tested into such relation that an area of the screen and an area of the surface fall wit-hin the field of thephotometerfor .comparison while sloping toward each other, whereby the' angle of observation between the photometer on the one hand and the areas under observation on the'other, is increased.

12. A device for measuring the coefficient of refiection of a surface, having a photometer, a diffusing screen and a frame for bringing the photometer, the screen and the surface to be tested into such relation that an area of the screen and an area of the surface fall within the field of the photometer for comparison while sloping toward each other, whereby the angle of observation between the photometer on lthe one hand and the areas under observation on the other, is increased, one of said areas falling on one side of the axis of the pho-tometer and the other area on the opposite side. of said axis.

13. A device for measuring the coefficient of reflection of a surface, having a photometer, a diffusing screen and a frame for bringing the photometer, the screenand the surface to be tested into such relation that an area of the screen and an area of the surface fall within the field of the photometer for comparison 'whilesloping toward each other, whereby a .dihedral angle is formed between the two areas, the axis of the photometer so projecting intosaid angle llO i that a plane through the axis of the photometer passes through the vertex of the dihedral angle.

14. A device for measuring' the coefficient of reflection of a surface, said device having 'a photometer, a diffusing screen for comparison and a frame for bringing the screen, the surface to be tested, and the photometer int-o such relation that an area of the screen and an area of the surface are brought within the field of the photometer, said frame constructed to maintain the screen and the surface in inclined relation to each other, thereby forming a dihedral angle between them, with the base toward the photometer, the photometer and the frame being so related that the said area of the surface which is brought within the field of the photometer is located at the position where the photometric error for a diffusing surface and the photometric error for a specular surface are approximately the same and of opposite signs.

15. A devicefor measuring the coefficient of reflect-ion of a surface, said device having a photometer, a diffusing screen for comparison and a frame for bringing the screen, the'surface to be tested and the photometer into such relation that an area of the screen and an area of the surface are brought within the field of the photometer, said frame constructed to maintain the screen and the surface in an inclined relation to each other, thereby forming a dihedral angle between them, one side of the frame forming the base of the dihedral angle, the photometer and the frame being so related that the center of the area of the surface is located between the mid point of the laltitude of the angle and the apex of the angle.

16. The process of measuring the coefficient of reflection of a surface which consists in bringing an area of a diffusing screen and an area of the surface to be measured within the field of a photometer, while the screen and the surface are inclined toward each other and taking a photometric reading.

17. The process of measuring the coefficient of reflection of a surface which consists in bringing an area of a diffusing screen and an area of the surface to be measured Within the field of the photometer, while the screen and the surface are inclined toward each other and toward the axis of the photometer and taking a photometric reading.

18. A device for measuring the coefficient of reflection of a surface, said device having in combination a photometer anda frame arranged to receive an illuminated surface and a surface to be tested, said frame placed in such relation to said photometer that the illuminated surface and the surface to be measured are both thrown into the field of the photometer for comparison when placed in said frame, and means interposed between the photometer andthe surfaces to increase the angle of observation through the photometer.

19. A device for measuringl the coefficient of reflection of a surface, said device having in combination a photometer and a frame arranged to receive an illuminated surface and a surface to be tested, said frame being placed in such relation to said photometer that the illuminated surface and the surface to be measured are both thrown into the field of the photometer for comparison when placed in said frame, said frame arranged to maintain said surfaces sloping toward each other and toward the axis of the photometer to increase the angle of observation through the photometer.

In witness whereof, I hereunto set my hand this 6th day of September, 1923.

FRANK A. BENFORD. 

